Natural gas and crude oil are extracted from the earth by drilling holes in the earth. A recent development in oil and gas extraction is horizontal drilling. In horizontal drilling, the drill is first inserted vertically into the earth until it reaches the required depth. The drill is then turned at an angle to penetrate horizontally into the geological layer. Thus the drill is able to access a wider area to increase the probability of striking a pocket of natural gas or crude oil. To further increase the flow of natural gas or crude oil, a fracturing fluid is injected at great pressure into the hole. The fracturing fluid permeates into the earth and opens up or creates additional flow channels for the flow of natural gas or crude oil into the hole. However, when the fracturing fluid is withdrawn, these flow channels are susceptible to collapse under the weight of the earth above them. Thus, some means is required to prop open the flow channels to prevent them from collapsing when the fracturing fluid is withdrawn.
A popular means of propping open the flow channels is by injecting “proppants” into the flow channels. Proppants are solids which can be carried by the fracturing fluid into the flow channels. When the fracturing fluid is withdrawn, the proppants remain wedged in the flow channel to “prop” up the flow channel. Thus for a proppant to be effective, it has to be strong enough to resist crushing by the earth above it and it should be large enough so the natural gas or crude oil can flow easily through the interstitial spaces between the proppants.
Proppants can be naturally occurring materials such as sand, quartz pebbles, etc. Alternately, they can be manufactured materials such as ceramic spheres. In yet another embodiment, proppants are coated with a resin to provide smooth yet tacky surfaces which can be easily inserted into the flow channels and remain in place after the fracturing fluid is withdrawn. A comprehensive description of various kinds of proppants is given in an article titled “Proppants: Where in the World” published in Journal of Petroleum Technology (April 2011).
All of these conventional methods and proppants have various disadvantages. For example, natural proppants are rarely uniformly sized and spherical. Thus they tend to clog and are difficult to insert into the flow-channels. Further, because they are not spherical, they are more susceptible to break under the pressure of flow channel walls.
Ceramic proppants are generally made by rotating wet clay in rotary mixers in a batch process until the clay forms somewhat spherical balls. It is difficult to control the size distribution of the product of this process. Since certain sizes of balls are more desirable, the non-desirable sizes of balls have to be rejected. This reduces the yield of the process. Further this process is capital and energy intensive because it requires many rotary mixers which have to be operated for a long time to form the balls.
U.S. Pat. No. 4,440,866 to Lunghofer, et al. describes a process for the production of sintered bauxite spheres by continuous spray-granulation of an aqueous binder-containing aluminous ore suspension to form granules which are subsequently sintered. Suitable starting materials include ores of high aluminum silicate content. However sintering produces proppants which are not very crush resistant and are therefore not commercially viable for use in natural gas and oil fracking operations.
U.S. Pat. No. 5,558,822 to Gitman et al. describes a continuous process for melting the surface of sand particles to make them more spherical and therefore more suitable for use as proppants. However, the process as described is not very practical as it is very energy intensive and therefore not economically viable.
There is therefore a need for a process and apparatus that will produce proppants of the desired size with very little rejection in an energy efficient manner.